Instabilities in spinal-column motion segments that may be caused by vertebral fractures, degenerative changes, etc. often require that the segments in question be fused. In order to ensure the immobilization of the segments to be fused as required for bone fusion, the corresponding segments are often stabilized with a fixation system. Fixation systems may either be inserted and anchored from the posterior, whereby the anchoring is done by means of bone screws in the pedicles, or from the anterior or antero-lateral, in which case the anchoring is done by means of bone screws in the vertebral bodies.
The quality with which fixation systems are anchored is heavily dependent on the quality of the bone structures. This is especially true of antero-laterally anchored fixation systems. The greater the degree of osteoporosis, the greater the danger that the bone screws will cut through the bone when subjected to even small loads. The use of thick screws reduces the risk of cutting through the bone. However, overly thick screws should be avoided lest there be excessive destruction of the bone structure in the vertebral body.
Known approaches in the related art for reducing the risk of the anchoring elements cutting through the bone are presented in DE 296 00 879 and in WO 00/10473. These publications deal basically with a hollow screw that is screwed into the vertebral body. The hollow screw does not need to displace a great deal of bone, because a peg of bone maybe left in place in the center of screw. However, when the hollow screw is being screwed in, the vascular supply to the peg of bone left in the center may be impaired, which may lead to complications, especially in osteoporotic bones. Also, in bones with still-functioning repair mechanisms, the hollow screw may be so heavily in-grown that it may be difficult to remove it if the area is to be inspected or, if removed, it will do serious damage to the bone (in some cases, for example, it has proved to be impossible to remove hollow screws inserted into the cervical vertebral column).
In connection with the surgical treatment of fractures in long bones, intramedullary stabilization techniques have been developed that, with modification, may also be successfully employed in the spinal column to solve the problem of anchoring anterior and antero-lateral spinal-column fixation systems. Intramedullary pins, for instance, may be used to splint fractured tubular bones by providing an intramedullary connection between the proximal portion of the broken tubular bone and its distal portion. Because of its geometry, however, the intramedullary pin can withstand only minor rotational and axial loads. This may not be potentially problematic as long as the fractured bone is able under axial load to maintain its height and the fracture is more or less diaphyseal. As soon as multi-fragment fractures arise, however, the intramedullary pin typically has to be anchored proximally and distally. In this way, the intramedullary pin can provide not only splinting but, as in the case of the spinal column, may act as a proximately and distally anchored longitudinal support that can transfer forces and moments at all levels from proximal to distal. In the case of the intramedullary pin, the anchoring implants may be screws that are run transversely through the bone and the intramedullary pin on the proximal and distal sides. In patients with osteoporosis and in cases where the fractures lie close to the joint, anchoring the intramedullary pin with screws is often not a satisfactory approach. Also, spiral-twisted blade-shaped implants known in the related art and as used in clinical practice are not particularly suitable for use on spinal column.